M4A to MP3 is about compatibility, not quality. M4A is what iTunes downloads, Apple Music exports and iPhone Voice Memos produce — AAC audio in an Apple wrapper — and while it sounds fine, older car stereos, cheap MP3 players and some Android apps simply will not play it. MP3 removes that wall.
Use it when an Apple-made audio file has to go somewhere un-Apple: a budget player, a fitness watch, a gym sound system, or a colleague on Windows. The two formats sound near-identical, so this is purely about making the track open on hardware that never learned to read M4A.